Fixes in xferfcn.py
Fixes in xferfcn.py

- Division by a scalar now works.
- _common_den() modified so that the numerator coefficient arrays have the same
  size as the common denominator.  This is done by padding with zeros.

Kevin K. Chen <kkchen@princeton.edu>


python-control/python-control,murrayrm/python-control,roryyorke/python-control